One and two dimensional barcodes are now in common use. The two dimensional barcode in particular has seen popular deployment in signs posted at various locations. A common encoding is the use of the QR code.
Typically, the 2d barcode encodes a URL. A common usage is for a user with a cellphone that has a camera to take a picture of the 2d barcode. Software on the cellphone decodes this to the URL. If the cellphone has (wireless) Internet access, it then goes out on the Internet to that address and downloads the webpage and displays it on the cellphone, in a browser. Whereby the user can interact with it as a standard webpage.
A barcode is preferred over the display of the URL in human readable text, because the latter needs the mobile user to read it and type it into her cellphone browser. The small size of the cellphone screen and the awkwardness of typing letters on the cellphone make the input of the URL error prone.
Such displays of 2d barcodes are usually in permanent form. For example, printed on a poster or piece of paper. It appears that libraries which use such barcodes do so in printed form. Also, the barcode would typically decode to a URL that is the library's URL. Here, the library, if it is part of a university or school, might have a subdomain below that of its parent organisation.